Green’s Dictionary of Slang

to have sexual intercourse with the man using rear entry.

1972		J. Wambaugh Blue Knight 114: Sexy little twist [...] a good body. I’d like to break her open like a shotgun and horsefuck her.
1996		(con. 1949) G. Pelecanos Big Blowdown (1999) 272: You ever horsefuck a fat girl, Joe?